1. Technical Field
This invention relates to a thin film magnetic head having a magnetoresistive (MR) read sensor for detecting signals recorded on a magnetic recording medium. More particularly, this invention relates to an improvement in construction of a low noise longitudinally biased MR read sensor.
2. Description of the Background Art
The use of a magnetoresistive (MR) sensor (also referred to as "transducer" or "head") to read signals recorded on a magnetic recording medium has been well established in the field of information storage and is believed to be the replacement for inductive read sensors. This is due to the fact that the MR read sensor has the following major advantages over the inductive read sensor:
1) the MR sensor's intrinsic noise is much lower than the inductive read sensor intrinsic noise thus providing improved signal-to-noise (S/N) performance, PA1 2) the MR sensor senses magnetic flux, not the rate of magnetic flux change, which means the magnetic signal recorded on the storage medium can be reproduced independently of the speed at which the medium is moving with respect to the sensor, and PA1 3) the MR sensor has bandwidth in gigahertz (gHz) range which allows areal storage density well in excess of 1 Gb/inch.sup.2 (1 gigabit per square inch). PA1 (1) it leaves undesirable thinned edges 140 around the circumference of end regions 110, of which inner thinned edges 160 behind trackwidth edge 170 of the MR sensor and near back edge 172 of the MR sensor are the most undesirable one. Indeed, during the ion milling, the end regions can be thinned to such an extent that the longitudinal material below the conductor layer is reduced in thickness to the point that the magnetic properties of the thinned end regions are changed, thus substantially impacting the longitudinal bias of the MR sensor; PA1 (2) the presence of thinned corner regions 165 result in a phenomenon known as current crowding which effect the amplitude of the signal read from magnetic storage medium; and, PA1 (3) ion beam milling does not completely remove the excess MR material; i.e., the material not covered by the photoresist (stencil) pattern, thus allowing a small remnant of MR material 150 to be left behind along the length of each inner thinned edge 160.
MR sensors currently fall into two broad classes: 1) anisotropic magnetoresistive (AMR) sensors and 2) giant magnetoresistive (GMR) sensors. In the AMR sensors, the resistance of the MR sensing element varies as a function of cos.sup.2 .alpha. where .alpha. is the angle between the magnetization and the direction of the sense current flowing in the sensing element. The sensing element is generally made of ferromagnetic material. In the GMR sensor, the resistance of the MR sensing element varies as a function of spin-dependent transmission of electrons between pinned and free magnetic layers separated by non-magnetic layers and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers. The magnetic layers are generally made of ferromagnetic material. GMR sensors using only two layers of ferromagnetic material separated by a layer of non-magnetic metallic material are generally referred to as spin valve (SV) MR sensors. The two layers of ferromagnetic material separated by a layer of non-magnetic metallic material are also referred to as "spin valve material".
FIG. 1 is a cross-section of a conventional thin film MR sensor 10, comprising magnetic shields 12 and 14, interlayer insulating material 13 and 15, MR sensor 18, soft magnetic material 16 for transverse biasing of the MR sensor, and leads 20 and 22. Leads 20 and 22 further comprise conductor materials and magnetic materials where the magnetic materials are used to longitudinally bias the MR sensor to eliminate Barkhausen noise.
As the demand for higher capacity storage devices continues to grow, it has become increasingly more important to produce MR read sensors small enough to read the data recorded in ever decreasing track widths at ever increasing recording density. One of the most prevailing solutions for meeting these requirements is described in commonly assigned U.S. Pat. No. 5,079,035 in which the disclosed MR sensor comprises an MR layer extending over substantially only a central active region and a hard magnetic bias layer provided in each of two passive end regions. Each end region further forms an abutting junction with the MR layer to produce longitudinal bias in the MR read sensor as shown in FIG. 2A.
Referring to FIG. 2A, there is shown a cross section of an MR read sensor 50 comprising an MR layer 62 deposited over substrate 66 and extending over substantially only the central active region 64 and a hard magnetic bias layer (also referred to as "longitudinal bias layer") 58 in each end region 52. Each end region 52 further includes a conductor layer 54 deposited over each magnetic bias layer 58. Each longitudinal bias layer 58 forms a contiguous (abutting) junction 60 with MR layer 62 to produce a longitudinal bias field in the MR read sensor 50.
In order to fabricate MR sensor 50 shown in FIG. 2A, a photoresist pattern is generally formed over both end regions 52 and MR layer 62. The pattern is then developed and the excess MR material is then removed, preferably by ion beam milling. Since end regions 52, which consist of both conductor material 54 and longitudinal bias material 58, are generally thicker than MR layer 62 in the central active region, and since the duration of ion beam milling is determined by the time it takes to remove the excess MR material, this means that not all of the exposed end regions' material are removed during the etching step. This in turn leads to the creation of thinned edges (shoulder) around the circumference of the end regions as well as leaving remnant MR material behind along a portion of the thinned edges and near the back edge of the MR sensor as shown in FIG. 2B.
FIG. 2B shows a top view of the MR sensor of FIG. 2A after ion beam milling and lapping steps comprising lead (magnetic bias and conductor materials) 70 in each end region 52 and thinned edge 74 along the circumference of each lead 70. Each thinned edge 74 further comprises inner thinned region 76 having a thinned corner region 78. MR sensor 50 further comprises trackwidth edge 80 (the trackwidth edge of the MR sensor is the surface, also referred to as the air bearing surface (ABS), which is in close proximity to the surface of the storage medium and is used for reading previously recorded information), back edge 82, and magnetic remnant material 84 along the length of inner thinned region 76 and near back edge 82.
The presence of the thinned edges around the circumference of the end regions as well as the remnant MR material are due to the method by which the prior art MR sensor is manufactured which is shown in FIGS. 3A-3F. FIG. 3A shows a top view of a step 100 of a process for fabricating an MR sensor prior to ion beam milling comprising MR material 120, end regions 110, and photoresist material 130. FIG. 3B is a cross-section of FIG. 3A along the line B-B' which is the region directly behind the active region of the MR sensor. FIG. 3C is a cross-section of FIG. 3A along the line A-A' which is a part of the active region of the MR sensor.
FIG. 3D shows a top view of step 112 of the process for fabricating the MR sensor after ion milling and photoresist removal steps where the duration of the ion beam milling has been determined by the time it would take to remove excess MR material. Step 112 comprises MR material 120, end regions 110, trackwidth edge 170, back edge 172, and thinned edges 140 of end regions 110. Each thinned edge 140 further comprises an inner thinned edge 160 having a thinned corner region 165. FIG. 3E is a cross-section of FIG. 3D along the line D-D' which is the region directly behind the MR sensor active region and FIG. 3F is a cross-section of FIG. 3D along the line C-C' which is a part of the active region of the MR sensor.
Referring back to FIGS. 3A-3F, it can be readily appreciated that the removal of the MR material by ion beam milling where the duration of ion beam milling is determined by the time it takes to remove excess MR material has the following major drawbacks:
A small remnant of MR material 180 is also left behind along the length of each outer thinned edge 185. Remnant MR material 150 can substantially alter and degrade the magnetic domain activities and increase the MR sensor noise, especially if the longitudinal bias material thickness has been reduced, thus reducing the signal-to-noise ratio.
Therefore, there is a great need for an invention that can substantially eliminate the longitudinal instability (unstable bias point) caused by the thinned edges of the end regions of the MR sensor and by the magnetic domain altering behavior and noise producing remnant MR material left behind.